<!DOCTYPE html>
<html lang="zh-CN">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>

<body>
    <script>
        function getSum() {
            let rest = +prompt('银行卡余额')
            let consume1 = +prompt('食宿消费')
            let consume2 = +prompt('当月消费')
            let sum = rest - consume1 - consume2
            alert(sum)
        }
        getSum()
    </script>
</body>

</html>